Slimy

sun, magnetic, spots, suns, lines, earth, solar and field

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— —6-7 1 ered fact of the impulsion of the solar rays probably affords a clew for an explanation of these and similar, phenomena. More than 40 years ago it was announced by Maxwell, as a result of his electro-magnetic theory of light, that light and heat emitted by the sun should exercise a very minute pressure on any object which they struck. Conclusion showed that this pressure was so slight that no apparatus then known was so delicate as to make it sensible. Within the last few years, however, . E. F. Nichols and others have succeeded in showing experimentally that on very finely divided mat ter this action of light can be observed and measured. It follows that particles below a certain size will be repelled by the sun's light with greater force than they are attracted toward it, and will thus be driven from the sun when in its neighborhood, or supported tempo rarily at a certain height above the sun.. Hale has proved, by spectroscopic methods similar to those employed in his discovery of magnetic fields in sun spots, that the entire sun is a mag net, with a field about 80 times as intense as the magnetic field of the earth and with its mag netic axis inclined about six degrees to the sun's axis of rotation.

The Sun's the light and heat which we receive from the sun are the source of all life on the earth, the important work is at once suggested to measure exactly how much radiant energy we receive from the sun in a given time, and especially, if possible, to find whether this is growing.greater or less, or if it varies from time to time. Until so late as 1905 the measurements were comparatively very crude, but the sensitiveness of the instruments employed has recently been so increased and observations with them have been so carefully and continuously carried on that this quantity has now been well determined.

For measuring the amount of heat received on a square unit of the earth's surface, the so-called pyrheliometer is employed an instru ment which presents a surface of icnown area to the solar rays, the rise in temperature due to the heating being communicated to a stream of water (in the best form), and measured by Prior to 1905 the true value of the con stant was in much doubt; numbers ranging from 1.76 to 4.10 were stated for it, and the average value 3.0 was frequently accepted. There can be no doubt that the value 1.95 is very near the truth, and this may be regarded as the best value now obtainable. It has been well established, however, especially by the recent work of Abbot, that this fundamental constant varies slightly and irregularly.: In 1919 a solar station was established at Calama, Chile, at which it is planned to make constant meas ures of the sun's radiation for several years, in conjunction with northern observations, with a special view of ascertaining, if possible, the law of this variation, and its effects upon ter restrial climates.

The Sun's In the year 1908, Prof. G. E. Hale of the Mount Wilson Observa tory, from an examination of the spectral lines in sun spots, discovered that around each spot there is a more or less powerful magnetic field. A powerful field will double many of the lines of the spectrum; a less powerful one will merely widen them. A few lines were found triple in sun spots, and afterward these same lines were found to become triple in the laboratory when viewed along the magnetic lines of force. Thus the lines of sun spots near the sun's limb tend to become triple, while those from spots near the centre of the solar disc are doubled merely. In many cases a pair of sun spots quite near together are found to have opposite polarity, and while in general the polarity of spots in the southern hemisphere is different from that in the northern, many cases of ex ception occur. It has, however, been well estab lished that the sun, like the earth, has a north and south magnetic pole; the inclination of the sun's magnetic axis has been determined, and the fact has been established that the magnetic pole is in slow rotation about the pole of the sun.

A curious relation is found by the study of magnetic storms on the earth. The latter con sist in occasional perturbations of the magnetic needle, which are very irregular in their char acter and are felt over the whole globe. They getn ally occur when there is an unusually bright aurora. Now, investigation shows that the number of these disturbances follows ex actly the period of the solar spots. During those years when the spots are most numerous magnetic storms are most frequent, and vice versa. The conclusion is that the sun spots and magnetic storms are due to the same cause. The sun's spots can be due only to something going on in the sun, and it follows that there must be some emanations from the sun which produce magnetic storms. Modern investiga tion has not been able to detect or define these emanations, though they are supposed to be electrically charged particles, shot out from the sun, and drawn in around the magnetic poles of the earth, where they give rise to the aurora. We have the strongest reasons to believe that neither the magnetic field of the sun as a whole nor the much more intense local fields in sun spots, produce any measurable magnetic effect at the distance of the earth.

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